The present disclosure relates generally to integrated circuits and, more specifically, to a method of forming bipolar transistors.
Complementary vertical bipolar processes require lightly doped N and P island to act as the collectors for the NPN and PNP bipolar transistors, respectively. A common method to form the complementary bipolar devices with respect to the substrate layer of one type, for example, N-type, is to form the opposite type collector by implanting P-type impurities through a collector mask and is usually followed by diffusion. The N base for the PNP transistor is formed after the collector diffusion using a separate base mask having a base aperture therein. N-type impurities are then implanted and diffused to define the base region. Additional masking steps are performed and P+ impurities are introduced to form the emitter region in the base and collector contact in the collector. A further mask is performed to provide an N+ base contact into the base region. The steps in a dielectrically isolated substrate is illustrated in FIGS. 1A-1C, which will be discussed below.
The process of the prior art generally requires a separate mask for the collector formation from the base formation. The use of the base mask and aperture to form an enhanced collector region in the collector to produce a high breakdown active device is described in U.S. Pat. Nos. 4,975,751 and 5,091,336. The enhanced collector region is the same conductivity type as the substrate. Adjusting a collector pattern to tune breakdown voltage in a bipolar transistor is shown in U.S. Pat. No. 6,555,894.
The present disclosure describes a method of forming a bipolar transistor which reduces the cost of manufacture by using the same mask to form the collector region in a substrate of an opposite conductivity type as to form the base in the collector region. More specifically, impurities of a first conductivity type are introduced into a region of a substrate of a second conductivity type through a first aperture in a first mask to form a collector region. Impurities of the second conductivity type are introduced in the collector through the first aperture in the first mask to form the base region. Impurities of the first conductivity type are then introduced into the base region through a second aperture in a second mask to form the emitter region.
When the region of the substrate is a dielectrically isolated region, the collector may be formed by diffusing introduced impurities for a sufficient time that they either reflect off the bottom of the dielectric isolation layer and spread laterally therefrom or extend at least to the bottom dielectric isolation layer. Also, the minimum dimension of the first aperture for the collector and base regions may be smaller than the depth of the formed collector region. The minimum dimension of the first aperture of the first mask is selected for a desired collector to base breakdown voltage. This allows tuning of the breakdown voltage.
These and other aspects of the present disclosure will become apparent from the following detailed description of the disclosure, when considered in conjunction with accompanying drawings.